Synthesizing a frequency by means of a VCO type oscillator is widely known. For this application, the VCO oscillator is associated with a phase-locked loop PLL. In such a circuit, a VCO oscillator control voltage is generated so as to make the output frequency of the oscillator match a reference frequency. The duly generated frequency is determined according to the type of use: mobile telephony (GSM, DCS, PSC, and other standards), VHF, UHF, and other similar communications.
Although the frequency synthesis circuit fabrication methods are increasingly fine and accurate, the output frequency of the oscillator is likely to vary according to a certain number of parameters linked, on the one hand, to the fabrication method and, on the other hand, to the conditions in which the oscillator is intended to operate. Such is, in particular, the case of a change of temperature which is likely to generate a variation in the value of the components of the oscillator, for example, the value of a capacitor in the oscillator circuit, but also to act on the value of the spurious components.
These changes are likely to induce a change in the output frequency of the oscillator, which is then likely to diverge from a required frequency.
When the frequency deviation is small, the phase-locked loop PLL acts on the oscillator control voltage so as to compensate the frequency deviation. However, when the difference between the output frequency of the oscillator and the reference frequency is so great that the PLL can no longer compensate it, the oscillator is no longer able to supply the desired frequency and malfunctions are likely to occur in the circuits in which it is incorporated.
There is therefore a need for improved systems and methods for calibrating an oscillator.